In a digital radio mobile system, the signal of a user is coded in digital form and the information relating to a certain number of users, for instance eight users, is multiplexed with time division, forming a frame where each user is assigned a time slot containing a train of bits (burst).
The signal thus formed will modulate the phase of a carrier wave, for instance with a constant envelope modulation or GMSK (Gaussian Minimum Shift Keying) phase modulation and each radio mobile receiver will extract only the relevant information from this flow.
The signals received in a radio mobile system show some distortions due to different causes such as propagation through multiple paths, Doppler effects, local oscillator drifts. These phenomena are emphasized further on in the event of digital signals having a relatively wide band, in the order of 200 KHz each channel. The receiver therefore is to provide for an equalization of the signal received in order to improve the quality of the communication.
According to the known technique, such equalization may be obtained by a transversal filter, with or without decision feedback, or else with a Maximum Likelihood Sequence Estimate (MLSE) utilizing a matched filter and a Viterbi processor. In lack of a proper equalization, the quality of the communication may be compromised even if the signal received is powerful enough.
On the other hand, the equalization techniques require certain power values in the signal received for their correct operation, while there is the need of keeping the transmission power within certain limits for various reasons, e.g. to prevent interferences between the various areas and to minimise the consumption and the dimensions of the mobile terminals. When the received signals have a power below the threshold value of the correction system, this is not able any longer to satisfactorily reconstruct the signal, thus causing a deterioration in the quality of the received signal.
It has been suggested that these inconveniences could be avoided by using space diversity techniques in digital radio mobile systems i.e. by using two or-more signals received from the same number of antennae positioned in such a way as to make these signals uncorrelated. The systems used are simple extensions to the digital case of the techniques known for the diversity reception of analog signals and are not devoid of inconveniences and restrictions.
One of these systems provides for the combination at the maximum power, i.e. the timing of the signals received and their subsequent addition, in order to maximize the signal/noise ratio of the combined signal. This system is not obviously capable of considering the distortion of the received signals which is a very important factor in multi-path propagation conditions. According to another technique, switching is provided on the antenna that in turn is receiving the signal at the highest power level.
The selection criterion can alternately be based on a quality estimate, but this requires a complete receiver for each antenna. The switching technique is also involving problems due to the switching transients which should be made to coincide with the inter-burst interval (i.e. between two subsequent bursts), considering also that the variation speed of the propagation conditions may be short as compared with the duration of the burst itself. Not even this technique may effectively allow for the distortion that the signal may show.
U.S. Pat. No. 4 328 585 discloses a receiver system for equalizing time-division digital signals received from a plurality of diversity channels or branches, providing for an estimate of the channel impulse response for each of the receiving branches, an adaptive matched filtering of each received signal, and the calculation of coefficients for the adaptive filtering and the equalizing of the received signals according to the adaptive filter technique, i.e. based upon the generation of an error signal and adaptive loops that lead to the convergence of the coefficients to optimum values.